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Re: Segnosaurs again (was: Quaesitosaurus and Nemegtosaurus)



In a message dated 96-12-10 17:41:20 EST, longrich@stardot.com writes:

> If we want to propose segnosaurs as non-theropod, we must ask 
> why a lineage not related to theropods would evolve 
> a long-fingered, big-clawed tridactyl manus with a semilunate carpal 
> bloc,  a booted pubis  and that birdlike v1 nerve opening when none of 
> these characters are known from outside the group? Should we even 
> seriously entertain the idea of nontheropod  segnosaurs when these 
> features have not yet been explained convincingly, 
> if even at all, in context of this hypothesis? 

I've not seen a semilunate carpal block in the type specimen of _Alxasaurus_,
despite examining the type specimen >in person< >specifically< for this
feature. In Russell & Dong's description of _Alxasaurus_, the carpal elements
that are supposed to make up this block are described as >separate<--take a
look at the figure on p. 2117 of Russell & Dong's description and tell me
just which of the two existing carpal elements is the semilunate. There IS NO
SEMILUNATE CARPAL in this earliest of known segnosaurs. If the illustration
of a semilunate carpal in _Therizinosaurus_ by Barsbold 1976 is correct
(debatable), then it must have developed independently in the group.

The supposedly birdlike cranial neurology is also completely unconvincing,
since the description (Clark et al., 1994) of the only known segnosaur
braincase, that of _Erlikosaurus_, contains anatomical errors and is
inadequately illustrated. Furthermore, the authors of that description do not
cite any neurocranial anatomy among their supposed theropod synapomorphies,
but rather cite certain neurocranial features as autapomorphies of
_Erlikosaurus_ and segnosaurs, >not< synapomorphies of birds and segnosaurs.
As I have pointed out in previous posts, the palate and basicranium of
_Erlikosaurus_ as illustrated by Clark et al. bear a >striking< overall
similarity to those of prosauropods, as illustrated by Galton in _The
Dinosauria_. (Note that it is the >basisphenoid< that is highly pneumatic,
and that the >parasphenoid< of _Erlikosaurus_ is not inflated as in
troodontids and ornithomimosaurs. Don't take my word for it; compare the
illustrations yourself.)

The feet of segnosaurs are completely unlike the feet of birds and theropods
in having four functional digits with the first digit high up on the
metatarsus proximal to the tarsus if not in direct articulation with it, and
in having four unbunched metatarsals. There are two distal tarsals,
articulating with metatarsals III and IV. >All< of these features occur in
prosauropods, >not< in any known theropods. Ask why, if segnosaurs are
theropods, their feet evolved an anatomy convergent with that of prosauropods
(of all groups!), whereas the feet of all other theropods from ceratosaurians
to birds remained virtually identical for more than 200 million years of
evolution.

Likewise, the shape of the segnosaur pubic boot is quite different from those
of theropods and could certainly have developed independently, as it did, for
example, in _Herrerasaurus_ (to which it bears a closer resemblance than it
does to any theropod pubis).

The manus and forelimb of all segnosaurs are highly derived and there are any
number of reasons that can explain the absence of digits IV and V besides a
putative close phyletic relationship between segnosaurs and theropods. The
manual digits of segnosaurs are >not< markedly elongate--only the claws
are--and the digits are not completely known in any specimen. So your
statement that segnosaurs had maniraptoran forelimbs is at best debatable and
most likely groundless.